The wireless communication of data between a vehicle and its tires is a major field of development in the automotive industry; numerous applications concerning both tire identification and the transmission of tire parameters such as tire pressure, tire temperature, tire rotational speed, or forces acting on the tire, have been developed.
One of the parameters limiting the development of such applications is the attenuation of the electromagnetic signal between the high frequency transmitter circuit and the receiving unit, which leads to a reduction of the transmission distance. A signal transmitted from or to the tire is attenuated by the cumulative effect of the rubber composition layers and steel belts. As mentioned in the patent application US 2002/0113692, the wheel rim, the steel belts in the tire and fillers in the rubber composition of the tire attenuate the signals so heavily that the antennas of the interrogator and of the responder must be arranged in close proximity to each other.
It has been suggested to take advantage the strong absorption of the electromagnetic waves in metallic components of the tire by using the latter as parts of the data transmission system: EP 0 389 406 describes a tire comprising a transponder that is capacitively coupled to a belt or breaker ply reinforced with steel cord; EP 0 875 405 discloses the use of a conductive wire in a jointless belt as the transmitting and receiving antenna of the transponder; U.S. Pat. No. 6,630,910 teaches that the coupling of a wave antenna to an inner steel belt can be direct, capacitive, or reactive.
Moreover, it has been observed that the electromagnetic field that penetrates into a tire is influenced by an electrically conductive belt and bent around the belt, thus yielding a distribution of the energy of the radio-wave field along the entire periphery of the belt. Patent application US 2003/0156067 therefore describes the use of the whole steel belt of a tire as an <<intermediate or auxiliary antenna >>. Such use of the belt opens interesting areas of application, but the very nature of the steel belts used in tires does not make them suitable candidates for all types of applications. If another type of antenna is to be used and has to be arranged close to the steel belt, the attenuation of the electromagnetic energy by the belt presents a serious problem and dramatically reduces the transmission distance mentioned above. In certain cases the distance is reduced to less than 50% of the distance that could be obtained with the same transmission circuit outside the tire.